1. Field of the Invention
The invention relates in general to protective relay apparatus for three-phase electrical power systems, and ore specifically to protective relay apparatus for detecting open phases, and single-phase-to-ground faults, in four-wire electrical distribution circuits.
2. Description of the Prior Art
A conventional distribution substation for supplying three-phase and single-phase loads includes a three-phase circuit breaker equipped with phase overcurrent relays, and sometimes a ground-current relay. Reclosers and fuses complete the protection of the distribution circuit. Coordination is necessary between these protective devices in order to isolate only the required portion of the distribution circuit when a fault occurs. Because of normal circit load unbalances, as well as normal cold-load pickup and switching surges, a ground relay, if used, must be set quite high in order to prevent nuisance tripping of the circit breaker. Broken conductors close to or touching the ground through high impedance paths may not produce enough current or circuit unbalance to operate the phase overcurrent protective devices, or the ground relay.
Prior art ratio ground relays have been solid state devices which develop zero sequence and positive sequence signals from sequence filter networks, which signals are rectified and compared with one another, and with threshold levels to determine if timing should be initiated, and if so, which one of certain timers should be activated. The timers provide the necessary coordination with other protective devices used in the distribution network.